The invention relates to a polyamide composition that is particularly suitable for the production of electrical and electronic components used in the so-called surface mounting technology, SMT, for printed circuits, which involves their being exposed to very high temperatures during the so-called reflow soldering process. The high temperatures used are obtained by, inter alia, infra-red radiation or hot-air circulation or a combination of both.
It goes without saying that the mechanical integrity of the component is to be retained at the high temperatures, which may increase to over 260xc2x0 C. There are various reasons for the use of high-melting polyamide compositions in SMT components.
These compositions generally contain fibre-reinforcing materials, often glass fibre and materials promoting flame retardancy.
In view of the very high processing temperatures, 300xc2x0 C. and higher, needed for the production of the SMT components from the high-melting polyamides, high demands are to be met in terms of the chemical stability of the flame retardants used and in particular flame retardants are suitable whose activity is based on incorporated halogen. Bromine-containing styrene polymers are very often applied.
In practice, in particular under tropical conditions, a disturbing phenomenon, blister formation on the surface of the soldered components, occurs when SMT components from the high-melting polyamides are reflow soldered. This phenomenon is ascribed to the fact that polyamides readily absorb water, which absorbed water will expand explosively in the component when the latter""s temperature is raised in a very short time, for instance by means of infrared radiation, to for instance 260xc2x0 C. Much research has therefore been aimed at reduction of the water absorption. JP-A-63-1959508, for instance, recommends drying the components to a water content of less than 0.8 wt. % before mounting them on the printed circuit board. This, however, necessitates an extra action and special logistics, while there must only be a short time between drying and mounting of the component. Factors interfering with the mounting process have a highly adverse effect.
Attempts have also been made, through incorporation of apolar polymers, or polymers having a low water absorption capacity, to reduce the water absorption capacity of the composition used, in particular of the aliphatic polyamide-4.6. To this end use has been made of, inter alia, aromatic polyesters, for instance polybutylene terephthalate, JP-A-03-190962, amorphous aliphatic polyamides with a long carbon chain, for instance polyamide-12, amorphous aromatic polyamides; for instance polyamide-6.6/6.T, JP-A-06-065502, and an aromatic polysulphone, JP-A-05-239344. In addition to sometimes negative effects on the mechanical properties of the components produced from these compositions, none of the compositions obtained proved to be an adequate solution to the blistering problem under all production conditions.
In particular under tropical conditions, 90% relative humidity and 35xc2x0 C., in a number of cases blistering is still found to occur, even though the water absorption capacity of the compositions has been reduced strongly.
The aim of the invention is a polyamide composition for use in electrical and electronic components suitable for SMT that exhibit an even greater resistance to blistering during the infrared or hot-air circulation reflow soldering process. The invention is aimed in particular at a polyamide composition which, in spite of a water content higher than 0.8 wt. %, does not give rise to blistering if processed in electrical and electronic components in the said reflow soldering processes.
This aim is achieved with a composition comprising
(a) a polyamide having a melting point of at least about 280xc2x0 C.
(b) a thermoplastic polymer having a melting point of lower than 230xc2x0 C.
(c) a halogen-containing organic compound which, when heated at 300xc2x0 C. for 10 minutes, generates at most 200 ppm, preferably at most 50 ppm, inorganic chlorine and at most 350 ppm, preferably at most 300 ppm, inorganic bromine.
(d) optionally a compound supporting the flame-retarding activity of (c)
(e) an inorganic reinforcement
(f) optionally other additives
The polyamide (a) may be either a homopolyamide or a copolyamide and be composed of repeating units derived from a diamine and a dicarboxylic acid or have been obtained by ring opening of a lactam.
Suitable diamines are aliphatic diamines, cycloaliphatic diamines and aromatic diamines. Examples are tetramethylene diamine, hexamethylene diamine, 2-methylpentamethylene diamine (2MP), 1,4-cyclohexane diamine (CHDA), 1,4-phenylene diamine (PDA) and p.xylylene diamine (PXDA). Suitable dicarboxylic acids are aliphatic, cycloaliphatic and aromatic dicarboxylic acids. Examples are adipic acid, pimelic acid, 1,4-cyclohexane dicarboxylic acid (CHDC), terephthalic acid (T) and isophthalic acid (I). Suitable homopolyamides and copolyamides include polyamide 4.6, polyamide-4.T, polyamide-4.6/4.T, polyamide-6.6/6.CHDC, polyamide-6.6/6.T, polyamide-6/6.6/6T and polyamide-6T/6I/2MPT. Several of these polyamides are commercially available under different trademarks. The polyamides can be obtained by means of polycondensation starting from the monomeric components or the corresponding nylon salts. These processes, which are known to one skilled in the art, are described in, inter alia, Encyclopaedia of polymer science and engineering, Vol. 11. p. 315 ff. (1988), ISBN 0-471-80943-8 (v.11) and the references quoted there.
Preferably, a polyamide with a melting point between 280xc2x0 C. and 340xc2x0 C. is chosen. At a higher temperature the stability of the constituent components is generally insufficient and processing by means of, inter alia, injection moulding presents technical problems.
Preferably, the melting point lies between 280 and 320xc2x0 C. Both the blistering behaviour and the mechanical properties are favourably affected by a higher molecular weight of the polyamide. One skilled in the art will aim to use a polyamide with the highest possible degree of polymerization that can still readily be processed.
For the thermoplastic polymer (b) in principle any polymer having a melting point below 230xc2x0 C. can be chosen. However, to retain the good physical properties of the composition, preference is given to a polymer that is at least to some degree compatible with polyamide (a). For this reason, polyolefins, for instance, are less suitable for use as polymer (b), but when the polyolefin is modified, for instance through grafting with maleic anhydride, so that polar groups are introduced that improve the compatibility with polyamide, polyolefins can be applied. The same holds for other apolar polymers that, through modification with inter alia carboxylic acid, amine and epoxy groups, are better compatible with polyamide. Preferably, the melting point is lower than 220xc2x0 C., even more preferably lower than 200xc2x0 C.
Particularly suitable are polyesters, copolyesters and polyamides. Examples of esters are polybutylene terephthalate, polyethylene terephthalate, polycarbonate and copolyether esters, for instance the copolyester derived from polybutylene terephthalate and polytetrahydrofuran. Suitable polyamides are polyamide 6, polyamide 8, polyamide 11 and polyamide 12. Preferably, the ratio of aliphatic C atoms: amide groups in the chain is at least 6, even more preferably at least 8.
Preferably, component (b) is very well dispersed in the matrix of (a). The best effects are obtained if the average cross-section of dispersed particles of component (b) in the composition in the solid state is less than 3 xcexcm, preferably less than 1 xcexcm for at least 90% of the particles .
The halogen-containing organic compound (c) is chosen from the group formed by halogen-containing organic compounds that are used as flame retardant for polyamide. The best known from this group are halogen-containing polymers. In view of the high processing temperatures and the complications that may present themselves at these temperatures due to the higher volatility of lower-molecular-weight products, preference is given to the polymeric halogen-containing compounds, of which the bromine-containing ones are the widest available and cause the fewest environmental problems.
At the high processing temperatures lower-molecular-weight halogen-containing compounds generally have a too high vapour pressure and often exhibit bleeding-out. The molecular weight Mw is therefore preferably  greater than 25,000, and even more preferably  greater than 30,000, while in general the molecular weight will not exceed 100,000 as in that case the processability will become problematic.
The use of halogen-containing polymers, in particular bromine-containing styrene polymers, which upon heating to 300xc2x0 C. generate at most 50 ppm inorganic chlorine and at most 350 ppm inorganic bromine, is known from JP-A-08-208978. In said patent a similar bromine-containing polystyrene compound is used to prevent the adverse effects of mould degradation and electrical corrosion by the inorganic chlorine or bromine. No mention is made of any positive effect of the low inorganic halide content to be generated on the blistering phenomenon and the combination with polymer (b.
The preparation of bromine-containing polymeric flame retardants suitable for use as component (c) is described, inter alia, in WO-A-91/13915, U.S. Pat. No. 5,369,202 and JP-A-05-287014.
The inorganic chlorine and bromine content generated upon heating to 300xc2x0 C. can readily be determined by heating a weighed amount of component (c) in a nitrogen flow and passing this nitrogen flow through a diluted solution of hydrogen peroxide (1%) in which the inorganic chlorine and bromine are absorbed and subsequently determined, for instance by means of ion chromatography or another standard technique. In this determination the sample is kept at 300xc2x0 C. for 10 minutes and is present in finely divided form.
When use is made of a polymer composition with a high processing temperature, it is advantageous for the said maximum amounts of inorganic chlorine and bromine to be generated only when heating at 320xc2x0 C. takes place, and even more advantageously to be generated only at a temperature of 340xc2x0 C.
The flame retardancy of the composition can be improved further through the presence of a component (d). In principle this can be any substance that has a synergistic effect with the halogen-containing flame-retardant compounds (c). In view of the very high processing temperatures, preferably inorganic compounds can be used for this. Examples may be metal oxides, for instance antimony oxide and the oxides of alkaline earth metals, metal hydroxides, for instance magnesium hydroxide and aluminium hydroxide, hydroxycarbonates and phosphorus- or boron-containing compounds. Antimony oxide and sodium antimonate are often used.
For the required stiffness the compositions generally contain a reinforcing fibre material, for instance glass fibre. The length and diameter of the reinforcing fibre material may vary within wide limits, the length for instance between 60 and 600xcexc and the diameter between 1 and 20xcexc. Other reinforcing fillers may also be present, for instance reinforcing materials in the shape of platelets, for instance mica, talc and clay. Preferably, these reinforcing materials are present in well-dispersed form.
The composition may further contain the customary additives, for instance stabilizers against the effect of high temperatures, UV light and hydrolysis, processing aids, for instance mould release agents and flow-promoting agents, colourants and pigments and toughness improvers.
Surprisingly, it has been found that an even better blistering resistance is obtained when also a compound containing acid imide groups according to the following formula is present in the composition. 
where R is H, or a group chosen from alkyl, aryl, alkaryl or aralkyl with 1 to 12 C atoms.
Examples of such compounds are styrene maleide imide copolymers, polyether imide and imidized polyacrylate. Preference is given to styrene maleide imide (SMI), which is commercially available; for instance Denka SMI(copyright) of Denki Ragaku Kogyo K.K., which is prepared by imidization of a styrene maleic acid copolymer (SMA).
Most surprisingly, the positive effect on blistering is not present upon addition of SMA.
A further advantage is that the flame retardancy is improved through addition of the acid imide containing compound so that in total fewer flame-retarding components in the composition suffice, which has a favourable effect on the mechanical properties.
The ratios in which the various components may be present in the composition varies within wide limits and is determined in part by the desired mechanical properties. In general, the composition lies within the following limits, the percentages being expressed as weight percentage relating to the total composition.
20-60 wt. % (a) preferably 23-55 wt. %
1-20 wt. % (b) preferably 2-15 wt. %
5-30 wt. % (c) preferably 8-25 wt. %
0-15 wt. % (d) preferably 2-12 wt. %
5-50 wt. % (e) preferably 10-45 wt. %
0-20 wt. % (f) preferably 0-15 wt. %
When use is made of a composition containing acid imide groups, its content is generally 0.5-20 wt. %, preferably 1-10 wt. %, even more preferably 1-5 wt. %.
The activity of the composition containing acid imide groups depends in part on the acid imide groups content.
The invention will now be elucidated with reference to the following examples and comparative experiments. Evidently, the invention is not limited to these and the various components, in particular (a), (b) and (c), can simply be replaced by substances exhibiting similar properties.
To keep the comparison between the effects of the various additions as strict as possible, in all examples, unless stated otherwise, the same type of glass fibre was used, 0.5 wt. % mould release agent and 0.33 wt. % of a thermal oxidative stabilizer.
After drying of the individual components the compositions were produced on a ZSK 25 twin-screw extruder. The dosing method employed affects the absolute levels of the properties. However, the examples and comparative examples in this application were all carried out in the same way, unless stated otherwise.
Of the composition test bars were produced on an Arburg CMD injection moulding machine, at a melt temperature of 310-350xc2x0 C., depending on the melting point of the high-melting polyamide.
Materials used:
(a)
1. STANYL(copyright) KS 200; low-molecular-weight polyamide-4.6 from DSM, Netherlands
2. KS 300; medium-molecular-weight polyamide-4.6 from DSM
3. AMODEL(copyright) AF 1113 from Amoco, USA; aromatic copolyamide 6.6/6.I/6.T
4. AF 4133 from Amoco, USA; aromatic copolyamide 6.6/6.T
5. Arlene(copyright) CH 230 from Mitsui, JP; aromatic copolyamide 6.6/6.T
6. Grivory(copyright) HTVS-3X2VO from EMS, CH; 6.6/6.T
7. Grivory(copyright) HTV -4X2VO; 6.6/6.T
(b)
1. polybutylene terephthalate. tm=225≅ C.
2. polyamide-11; Rilsan(copyright) B MNO from Elf Atochem, France. tm=185xc2x0 C.
3. polyamide-12; X-1988 from Hxc3xcls, Germany, tm=180xc2x0 C.
4. polyethersulphone; Victrex(copyright) 3600 from ICI, UK, tg=179xc2x0 C.
(c)
1. Pyro-check(copyright) 68 PB; brominated polystyrene from Ferro, USA, inorganic Cl=730 ppm and Br=760 ppm at 320xc2x0 C.
2. PDBS(copyright) 80; polydibromostyrene from Great Lakes, USA. Cl=3 ppm and Br=74 ppm
(d) Sb2O3; antimony trioxide
(e) glass fibre; diameter 10 xcexcm, length 3 mm
(f) SMI; Denka(copyright) SMI MS-NA from Denki Kagaku, Japan
The tensile tests were carried out in conformity with ASTM D-638.
The flame retardancy was determined according to the method of US Underwriters Laboratory (UL 94). Thickness of the test bars 0.8 mm.
The water absorption was measured on the basis of the weight increase of the test bars upon exposure to the environment as specified.
A first series of experiments was carried out with the following compositions: